To practise Hadoop you can use below ways to generate the big data (GB),So that you can get the real feel/power of the Hadoop.
From clearbits.net, you can get quarterly full data set of stack exchange so that you can use it while you are practising the hadoop . it contains around 10 GB data.
grouplens.org collected different rating data sets ,you can use it for practicing the hadoop.
If you have Hadoop installed on your machine,you can use the following two ways to generate data.
3.hadoop jar /usr/lib/hadoop-0.20-mapreduce/hadoop-examples.jar randomwriter /random-data
generates 10 GB data per node under folder /random-data in HDFS.
4.hadoop jar /usr/lib/hadoop-0.20-mapreduce/hadoop-examples.jar randomtextwriter /random-text-datagenerates 10 GB textual data per node under folder /random-text-data in HDFS.
path of hadoop-examples.jar may change as per your hadoop installation.
5. Amazon provides so many data sets ,you can use them.
6. Check answers of the same question on stackoverflow
7.From University of Waikato ,many data sets available for practicing machine learning.
8.See answers for the similar question on Quora.
If you know any free data sets ,please share in comments